1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for providing a seismic source with a positive reflection plate while being towed in water by a vessel.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon reservoirs.
A traditional system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array (or simply a source) 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data.
The seismic source array 16 includes one or more individual source elements. Such individual source elements may include an air gun, water gun, a vibratory source, etc. The individual source elements may be grouped in one or more sub-arrays. The totality of sub-arrays forms the seismic source. Traditionally, a seismic source has three sub-arrays.
FIG. 2 shows a vessel 100 towing two source arrays 110a and 110b (it is also possible to tow only one source array or more than two source arrays but, for simplicity, the novel features are discussed with regard to two source arrays) and plural streamers 120. The streamers 120 include receivers 121 that are configured to record signals generated by the source arrays and reflected by the subsurface. The streamers are connected to the vessel through lead-ins 122 while the source arrays 110a and 110b are connected through cables 112 to the vessel 100. Each source array 110a or 110b may include sub-arrays 114, each sub-array having plural individual source elements 116. Deflectors 140 are provided on the sides of this arrangement to maintain a transverse distance (relative to the path of the vessel) between the streamers 120. The deflectors 140 are connected to the vessel 100 via wide tow cables 142, and spread ropes 144 are used to separate the streamers from each other. It is noted that the terms “rope” and “cable” and “wire” are used sometimes interchangeably in this document. Thus, these terms should not be construed in a narrow sense but rather as those skilled in the art would expect. The number of streamers or individual source elements is exemplary and not intended to limit the applicability of the novel concepts.
The traces recorded by the receivers of the streamers are affected by an effect that limits the accuracy of inferring the structure and composition of the subsurface. This effect, known as source ghosting, arises because water has a higher density and higher propagation velocity of pressure waves than the air above the water surface. Because of these factors, when the source array is activated, acoustic energy is generated and propagates from the source array along various (up, down, left, right, etc.) directions that originate at the source array. In other words, part of the acoustic energy travels downward and interacts with the subsurface while another part travels upward to the water surface. Because the water surface acts as a mirror, this energy is reflected back into the water and travels downward to also interact with the subsurface. The water-air interface reflected acoustic energy is delayed in time and is also shifted in phase by about 180 degrees from the directly downward-propagating acoustic energy. The acoustic energy that is interface-reflected and travels downward is commonly known as the “ghost” signal.
The ghost signal interferes with the directly downward-propagating signal, causing constructive interference in some parts of the frequency band and destructive interference in other parts of the frequency band. The destructive interference causes the apparition of notches in the spectrum recorded by the recorders. The frequencies of these notches are related to the depth at which the source arrays are disposed. The presence of the notches reduces the available spectrum. Thus, source ghosting reduces the effectiveness of the recorded spectrum, which is undesirable.
There are various methods in the field for dealing with source ghosting. Most of these methods involve using additional streamers, or firing the sources following a certain recipe, or having the sources placed at various depths into the water, and/or using time-consuming mathematical algorithms for removing the ghost signal during processing. However, all these methods result in a cost increase in the seismic survey, which is undesirable in a competitive market.
Accordingly, it would be desirable to provide a source array that removes (partially or totally) the ghost effect without substantially increasing the cost of a seismic survey.